Render is grainy with GPU
I think I found my problem. It is related to the shader "volume scattering" that I put in the world settings in the "volume" if I remove this option my CPU rendering become identical to my GPU rendering. I found on several website people who say that there is a problem with the "volume scattering" and the CUDA.

https://blenderartists.org/t/cycles-gpu-renderer-cant-render-volume-scatter/647962

https://developer.blender.org/T50808

https://blenderartists.org/t/volume-scatter-with-gpu-worse-than-with-cpu-in-cycles/651603